build: Silence libtoolize warnings

When running the autogen.sh script, libtoolize complains thusly:

	libtoolize: Consider adding `AC_CONFIG_MACRO_DIR([m4])' to configure.ac and
	libtoolize: rerunning libtoolize, to keep the correct libtool macros in-tree.
	libtoolize: Consider adding `-I m4' to ACLOCAL_AMFLAGS in Makefile.am.

Silence the warnings by following libtoolize's advice.

Signed-off-by: Thierry Reding <treding@nvidia.com>
diff --git a/configure.ac b/configure.ac
index 16e813d..b4511fc 100644
--- a/configure.ac
+++ b/configure.ac
@@ -18,6 +18,7 @@
 
 AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_HEADERS([config.h])
+AC_CONFIG_MACRO_DIR([m4])
 
 AC_USE_SYSTEM_EXTENSIONS
 AC_SYS_LARGEFILE